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Sperm aim for the sweet smell of success

21 January 1995

By Peter Aldhous

MAMMALIAN sperm navigate by sense of smell, according to a team of US biologists. The researchers have found that rat sperm carry two types of molecule which are also found in cells lining the nose. These are the same molecules that allow nasal cells to respond to odours. The discovery may ultimately lead to the development of new contraceptives which would disable sperm by blocking their ability to smell.

In the past couple of years, researchers in Belgium have found that some of the receptor molecules lining the nose are also present in testes and sperm. In nasal cells, these receptors bind to odorous molecules entering the nose, setting off a chain of chemical events that leads to the sense of smell. This suggested sperm may use these receptors to detect chemicals given off by egg cells, and so help guide the sperm to the egg. Until now, however, there has been no hard evidence showing that the receptor molecules to this job.

A team led by Solomon Snyder of Johns Hopkins University in Baltimore, Maryland, has now pinpointed the location of the receptors. They also searched for a second group of molecules called desensitisation proteins, which temporarily stop the receptor cells from responding to an odour. These proteins cause a smell to become less strong if we continue to sniff at it.

The keys to the researchers’ success were antibodies which recognise and bind to specific odour receptors or desensitisation proteins. They mixed several of these antibodies, one by one, with rat sperm, and then looked to see where the antibodies accumulated. Because the antibody molecules had been attached to fluorescent dyes, they were easy to see under the microscope. The researchers found that antibodies recognising both the receptors and the desensitisation proteins bound to the midpiece of rat sperm cells. This region is packed with mitochondria which generate the energy that sperm need to swim (Molecular Medicine, vol 1, pl2).

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The fact the sperm cell receptor molecules are found alongside desensitisation proteins – just as they are in the nose – strongly suggests that they detect odours, says Snyder. And the precise location of both, close to a sperm cell’s “engine room”, also fits this role. Snyder believes a sperm cell uses the receptors to detect chemicals released by egg cells. The receptors presumably then pass this information to the sperm’s mitochondria, which power the cell towards the source of the odour. Snyder’s team is now trying to confirm this picture by experimenting with purified extracts taken from crushed rat eggs. The researchers hope to find chemicals that bind to the odour receptors and attract sperm.

Eventually, it may be possible to identify molecules which disable the odour receptors, and so prevent sperm from finding their way towards egg cells. In theory, says Snyder, such molecules could be used to develop a male contraceptive pill which would knock out the receptors before sperm are ejaculated. This may, however, have an unusual side effect&colon; men taking the pills could lose their sense of smell.